Spheroidal polymer beads in the size range from about 1 to 300 μm in diameter are useful for a variety of applications. For example, such polymer beads have been employed for various chromatographic applications, as substrates for ion exchange resins, seeds for the preparation of larger sized polymer particles, calibration standards for blood cell counters, aerosol instruments, in pollution control equipment, and as spacers for photographic emulsions, among other uses.
Unfortunately, however, the preparation of uniformly sized polymer beads using known methods is often not suitable for large-scale production. Typically, polymer beads can be prepared by suspension polymerization by dispersing an organic monomer phase as droplets in a vessel equipped with an agitator and an aqueous phase in which the monomer and resulting polymer are essentially insoluble. The dispersed monomer droplets are subsequently polymerized under continuous agitation (see, for example, U.S. Pat. Nos. 3,728,318; 2,694,700; and 3,862,924). Polymer beads are also manufactured by “jetting” liquid organic monomer mixtures through capillary openings into an aqueous phase or gaseous phase. The monomer droplets are then transported to a reactor where polymerization occurs, as described, for example, in U.S. Pat. Nos. 4,444,961; 4,666,673; 4,623,706; and 8,033,412. However, these conventional methods, such as stirred batch polymerization, often produce bead products exhibiting large particle size distributions, primarily due to problems of non-controllable coalescence and/or breakage of the suspended monomer droplets. Existing jetting methods also suffer from high cost and low output for particle size products of less than 300 μm. For example, plate jetting methods have low overall productivity and are limited by large energy losses during the vibration generation step. Moreover, methods which require jetting into a gaseous media demand very sophisticated equipment and complex methods for polymer formation. The use of cross-flow membranes for the generation of fine droplets using a metal or glass sintered or electro-formed membrane is appropriate for small scale applications but is unfeasible for commercial operation. Further, the low productivity per unit area of the cross flow membrane requires complex and bulky equipment which is unreliable and demands high capital and operating costs. Metallic plate or can-shaped membranes, preferably of nickel or nickel-plated are desirable for use in vibration jetting. However, while such plates are relatively long-lived, over time they are known to experience wear during use. Such wear alters the configuration and geometry of the membrane pores (or “through holes”; as used herein the terms pores and through holes are interchangeable), and increases non-uniform drag on the monomer, resulting in inconsistent, non-uniform bead production and increased energy costs. Therefore, an object of the present invention is to provide a metallic membrane with a durable surface, providing a long service life without deterioration. Other jetting method for producing polymer beads are described in U.S. Pat. Nos. 9,028,730 and 9,415,530.